| Summary: | 碩士 === 國立中興大學 === 生命科學系所 === 103 === The milkfish (Chanos chanos) is a euryhaline and a popular aquaculture species in Taiwan. However, the cold snap in winter usually leads to serious mortality. Gill Na+, K+-ATPase (NKA) activity was decreased in cold-exposed milkfish. However, expression of NKA α1 subunit did not change under low temperature. The results showed that NKA activity might be reduced because of changes in NKA β1 subunit expression. This study thus aims to explore the expression of branchial NKA β1 subunit and improve the NKA activity via cortisol injection in fresh water (FW) milkfish when exposed to low-temperature.
From the transcriptome database of milkfish derived from next generation sequencing (NGS) analyses, a NKA β1-like sequence, Locus_38947, was found. The phylogenetic tree analysis indicated that this sequence was classified into the group between the NKA β1 and the NKA β233 isoform. Since the prediction of transmembrane domain showed that the glycine zipper was absent in NKA β233, the Lcous_38947 was considered to be NKA β1. The highest expression of NKA β1 subunit was found in gills compared to the other tissues or organs. In the fresh water (FW)-acclimated milkfish branchial NKA β1 expressed higher in mRNA and protein abundance than the seawater (SW)-acclimated group. The NKA β1 was observed in ionocytes and interacted with NKA α1 in gills. On the other hand, the plasma cortisol contents as well as gill 11β-hydroxysteroid dehydrogenase 1 (11β-hsd1) and nka β1 mRNA abundance were decreased in low-temperature FW milkfish. In the low-temperature SW group, however, the plasma cortisol contents as well as gill 11β-hsd1 and nka β1 mRNA abundance was increased. After cortisol injection, NKA activity, NKA β1 subunit expression, and 11β-hsd2 abundance were increased in gills of FW milkfish. In contrast, the 11β-hsd2 abundance was also induced after cortisol injection while NKA β1 subunit expression was constant in SW milkfish gills. Taken together, we suggested that the NKA activity of FW milkfish was decreased because of reducing 11β-hsd1 abundance, and the fish was unable to transform more endogenous cortisol to regulate gill NKA β1 expression in low-temperature environments. The exogenous cortisol could rescue the NKA activity in gills of cold-exposed FW milkfish . Meanwhile, cortisol did not affect gill NKA β1 expression in cold-exposed SW milkfish. These results indicated that the SW and FW milkfish utilized different mechanisms to confront the environments with lower temperatures.
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